1. Field of the Invention
The present invention relates to friction welding and, more specifically, to a tool and method for forming joints between workpieces by friction stir welding.
2. Description of Related Art
The general concept of friction stir welding is generally described in U.S. Pat. No. 5,460,317 and “An Outsider Looks at Friction Stir Welding” (Federal Aviation Administration July 2005), both of which are incorporated by reference. Conventional friction stir welding is a process of welding using the frictional heat generated by a non-consumable tool to join workpieces together. The friction stir welding tool includes a pin or probe that is inserted into the joint and a shoulder that is applied against an upper surface of the workpiece. The pin and shoulder rotate to generate friction needed to create a plasticized region along the joint for the welding operation.
For conventional friction stir welding operations, the weld tool shoulder is usually a concave or flat surface that rests against the workpiece. U.S. Pat. No. 6,669,075 also describes a friction stir welding tool having a tapered shoulder. The taper extends from the outer edge of the shoulder downward toward the pin. The '075 patent also describes a tool in which there are multiple concentric shoulder sections tapered at different angles, as well as a tool having an inner flat concentric shoulder section and an outer tapered concentric shoulder section.
Conventional friction stir welding tools have also employed shoulders that contain spiral or concentric grooves. These grooves in the shoulder are commonly known in the friction stir welding art as a “scrolled shoulder.” For shoulders comprising a flat surface, the grooves conventionally extend along the entire shoulder surface from the pin toward the outer periphery of the shoulder. For shoulders including one or more shoulder sections, such as those in the '075 Patent, the grooves extend along the entire surface of the shoulder section.
The present invention relates to the surprising discovery that the improved friction weld may be achieved when the scrolled shoulder does not cover the entire shoulder surface. By forming a scroll-free region in the shoulder section, an improved fiction stir weld is achieved. It is theorized that this scroll-free region functions as a “wiper” to wipe the workpiece(s) substantially clean of debris formed during the friction stir welding process and force the debris (or “flash” as it is sometimes called) to the outside of the welding area.